From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
"Frédéric Weisbecker" <fweisbec@gmail.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Andrew Morton" <akpm@linux-foundation.org>
Subject: Re: [GIT PULL] NOHZ updates for v4.6
Date: Tue, 15 Mar 2016 10:49:21 +0100 [thread overview]
Message-ID: <20160315094921.GB7943@gmail.com> (raw)
In-Reply-To: <20160315084220.GR6344@twins.programming.kicks-ass.net>
* Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Mar 14, 2016 at 07:44:14PM -0700, Linus Torvalds wrote:
> > On Mon, Mar 14, 2016 at 5:32 AM, Ingo Molnar <mingo@kernel.org> wrote:
> > > +/**
> > > + * fetch_or - perform *ptr |= mask and return old value of *ptr
> > > + * @ptr: pointer to value
> > > + * @mask: mask to OR on the value
> > > + *
> > > + * cmpxchg based fetch_or, macro so it works for different integer types
> > > + */
> > > +#ifndef fetch_or
> > > +#define fetch_or(ptr, mask) \
> > > +({ typeof(*(ptr)) __old, __val = *(ptr); \
> > > + for (;;) { \
> > > + __old = cmpxchg((ptr), __val, __val | (mask)); \
> > > + if (__old == __val) \
> > > + break; \
> > > + __val = __old; \
> > > + } \
> > > + __old; \
> > > +})
> > > +#endif
> >
> > This is garbage.
> >
> > This macro re-uses the "mask" argument potentially many many times, so
> > semantically it's very dubious.
>
> So the below cures that; but do we want to maybe pull this back into
> sched.c and expose a version that operates on a fixed type instead?
So there are other problems with the macro - see the patch I just sent that tries
to fix all of them.
> Although with xchg() and cmpxchg() we've already set a precedence for
> multi-width operators.
Yes, and if we name the new API 'xchg_or()' as I did it in my patch then it all
nicely fits into the existing scheme.
Thanks,
Ingo
next prev parent reply other threads:[~2016-03-15 9:49 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-14 12:32 [GIT PULL] NOHZ updates for v4.6 Ingo Molnar
2016-03-15 2:44 ` Linus Torvalds
2016-03-15 8:42 ` Peter Zijlstra
2016-03-15 9:49 ` Ingo Molnar [this message]
2016-03-15 9:32 ` [PATCH] atomic: Fix bugs in 'fetch_or()' and rename it to 'xchg_or()' Ingo Molnar
2016-03-15 10:50 ` Peter Zijlstra
2016-03-15 12:08 ` Ingo Molnar
2016-03-15 12:42 ` Peter Zijlstra
2016-03-15 11:06 ` Peter Zijlstra
2016-03-15 11:59 ` Peter Zijlstra
2016-03-15 12:01 ` Ingo Molnar
2016-03-15 12:32 ` Ingo Molnar
2016-03-15 12:37 ` Ingo Molnar
2016-03-15 13:17 ` Peter Zijlstra
2016-03-15 12:21 ` [PATCH v2] " Ingo Molnar
2016-03-15 13:26 ` Peter Zijlstra
2016-03-16 8:04 ` Ingo Molnar
2016-03-16 8:29 ` Peter Zijlstra
2016-03-15 17:08 ` Frederic Weisbecker
2016-03-16 8:14 ` Ingo Molnar
2016-03-17 0:54 ` Frederic Weisbecker
2016-03-15 16:18 ` [PATCH] " Linus Torvalds
2016-03-15 9:53 ` [PATCH] nohz: Change tick_dep_mask from 'unsigned long' to 'unsigned int' Ingo Molnar
2016-03-15 12:15 ` Ingo Molnar
2016-03-15 16:30 ` Linus Torvalds
2016-03-15 17:28 ` Frederic Weisbecker
2016-03-15 17:36 ` Linus Torvalds
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